Aim. Aim of the present study was to compare in vitro the labelling efficiency (LE) and cell viability (TBE) of autologous leukocytes labelled with 99mTc-SnF2 and 99mTc-HMPAO, and to evaluate the quantity and quality of spontaneously released 99mTc (SR) from labelled cells at several time points after labelling.Methods. A total of 14 patients with different diseases and 18 normal subjects were included in this study. A blood sample was collected from each patient; purified autologous leukocytes were divided into 2 samples and labelled with 99mTc-SnF2 and 99mTc-HMPAO. LE was evaluated at the end of labelling and TBE and SR were evaluated at 10 min and 1 h, 2 h and 4 h after labelling.Results. LE of 99mTc-SnF2-WBC was higher than 99mTc-HMPAO-WBC (61.2±18.7% and 43.3±11.3; p<0.0001) and we found an inverse correlation between blood glucose and labelling efficiency for both methods (p=0.02). Minimal differences were also observed between 2 methods after 10 min and 1 h, as far as the cell viability is concerned. The percentage of radioactivity spontaneously released from 99mTc-SnF2-WBC was significantly higher compared to 99mTc-HMPAO-WBC at each time point. Radioactivity released from labelled cells was predominantly 99mTc-SnF2 and 99mTc-HMPAO with few free 99mTc (<20%).Conclusion. Both radiopharmaceuticals are not toxic for WBC. Labelling with 99mTc-SnF2 give a higher LE than with 99mTc-HMPAO; however, radiolabelled colloids are more released from labelled cells over a period of 4 h. While 99mTc-HMPAO is physiological excreted into gastrointestinal tract, 99mTc-SnF2 can be re-uptaken in vivo by reticulo-endothelial cells of liver and spleen. These findings suggest that 99mTc-SnF2-WBC might be better than 99mTc-HMPAO-WBC for studying inflammatory bowel diseases.